log/summarize: set undefined strings as empty str
log/summarize: set undefined strings as empty str

avoids undef warnings

msimerson/qpsmtpd,sieutruc/qpsmtpd,priyadi/qpsmtpd,smtpd/qpsmtpd,smtpd/qpsmtpd,sieutruc/qpsmtpd,salvis/qpsmtpd,salvis/qpsmtpd,sieutruc/qpsmtpd,msimerson/qpsmtpd,priyadi/qpsmtpd,priyadi/qpsmtpd